As a method for controlling a rotational speed of a disc-shaped recording medium when data is recorded and reproduced, there are known a constant angular velocity (CAV) control method in which data is recorded and reproduced at a constant rotational speed and a constant linear velocity (CLV) control method in which data is recorded and reproduced at a constant linear velocity. Comparing these two kinds of control methods, in the case of the constant angular velocity control method, the revolution rate per unit time is constant so that the speed can be controlled relatively easily. Also, control data such as a track address or the like are recorded on the same position in the diametrical direction so that the address search can be carried out with ease. On the other hand, in the case of the constant linear velocity control method, the rotational speed must be changed in response to the position of an optical pickup in the diametrical direction so that a speed control system becomes complicated. However, the recording density can be made constant and therefore the amount of recording information can be increased over that of the constant angular velocity control method. However, the recording position of the control data such as the track address or the like is changed at every track. There is then the disadvantage such that the address search becomes cumbersome.
In general, in most of the magneto-optical disc having large data recording capacity, in order to search necessary data with ease, data is recorded and reproduced by the constant angular velocity method in which the address search can be made with ease. However, as described above, the constant angular velocity control method is small in recording capacity as compared with the constant linear velocity control method. Therefore, as a method of increasing the recording capacity when data is recorded by the constant angular velocity control method, there is proposed a zoning recording method.
FIGS. 1 and 2 show the condition such that the recording capacity is increased by this zoning method. FIG. 1 shows the example that the zoning recording is effected under the condition that the magneto-optical disc is divided into three zones A, B and C. FIG. 2 shows the example such that the zoning recording is effected under the condition that the magneto-optical disc is not divided into the zones (in the case of the constant angular velocity control method). In these examples, data is recorded on the discs in the form of pits. When the zoning recording is effected as shown in FIG. 1, the recording linear densities of innermost peripheral tracks T11, T21 and T31 of the respective zones A, B and C are set to be the same. Within the respective zones A, B and C, the constant angular velocity control is effected (if the zone is changed, then a recording data clock frequency is changed) so that the recording linear density is lowered in the tracks of the outer peripheral side within the zone. If the magneto-optical disc is divided into a plurality of zones as described above, the linear recording density of one track can be returned to the highest condition each time the zone is changed. Thus, as compared with the case where the linear recording density is sequentially lowered to the outermost peripheral track when the magneto-optical disc is not divided into the zones as shown in FIG. 2, the recording capacity can be increased considerably. That is, although the outermost peripheral track provided when the magneto-optical disc is not divided into the zones is extremely low in linear recording density, if the magneto-optical disc is divided into a plurality of zones, then a sufficiently high linear recording density can be obtained even in the outermost peripheral track. In this case, by increasing the number of zones, the recording capacity can be increased much more.
However, when the zoning recording is effected as described above, the control operation such as to extract a clock or the like in the playback mode must be switched at every zone. Therefore, in order to readily extract a reproducing clock or the like when the zone is switched, it is proposed that reference data that is used to detect the quality of a recording signal is recorded at every predetermined interval. However, when the above-mentioned zoning recording is effected in an optical disc of a so-called sample servo system in which servo control data are previously recorded in alignment with the diametrical direction of the disc in the form of convex and concave pits, a data capacity within one segment segmented by servo control data is made different between the respective zones so that the above-mentioned reference data and the recording data are continuously recorded within one segment. However, in the playback mode, after the reference data is reproduced, the playback control is effected on the basis of this reference data so that the playback of the recording data recorded immediately after the reference data becomes unstable.